In a typical wet process for manufacture of acetylene from calcium carbide, particles of calcium carbide are introduced to an excess of water in a reactor vessel on a continuous or semi-continuous (on/off) basis. Water is added continuously to the reactor and acetylene and a hydrated carbide lime slurry are withdrawn from the reactor on a continuous basis. A system is provided for stirring the contents of the reactor to mix the calcium carbide with the water and to maintain a more-or-less uniform slurry of hydrated carbide lime. Because the reaction of calcium carbide with water is exothermic, the temperature must be controlled, typically by the rate at which fresh water is added. A greater rate of water addition results in cooler temperatures and a lesser rate of water addition results in warmer temperatures.
There are several undesirable characteristics of the process of the prior art. These in summary are:
1. Some acetylene yield is lost through the premature discharge of unreacted calcium carbide. PA1 2. Some acetylene yield is lost through the solubility of acetylene in the large volume of water passing through the reactor. PA1 3. Operational difficulties occur due to large, solid inert particles that enter the system and interfere with the stirring mechanism and discharge pumps and valves. PA1 4. The overall process efficiency is reduced because a low concentration of hydrated lime in the discharge stream results in low contact times in the reactor and poor use of the reactor space. PA1 5. The hydrated carbide lime value is reduced due to the presence of granular impurities and variable hydrated lime concentrations.
These characteristics are described further as follows:
Discharge of Unreacted Carbide (breakthrough)
In a typical reactor configuration the reaction kinetics are approximated by a Constant Flow Stirred Tank Reactor (CFSTR). According to Levenspiel, Chemical Reaction Engineering, 2nd Edition, John Wiley and Sons, Inc. 1972, Chapter 5, page 97. "one type of! ideal steady-state flow reactor is called the mixed reactor, the backmix reactor, the ideal stirred tank reactor, or the CFSTR (constant flow stirred tank reactor) and, as the name suggests, it is a reactor in which the contents are well stirred and uniform throughout. Thus the exit stream from this reactor has the same composition as the fluid within the reactor. We refer to this type of flow as mixed flow, and the corresponding reactor, the mixed reactor, or the mixed flow reactor." Most, if not all, commercially practiced wet acetylene processes approximate the CFSTR configuration.
In the CFSTR there is unreacted carbide that is mixed throughout the reactor. The particle size of the unreacted carbide varies from larger, recently introduced particles to smaller less-recently introduced particles that are nearing the completion of reaction. In an ideal CFSTR some of these smaller unreacted particles will be discharged with the hydrated lime slurry. Any acetylene generated by a particle after it has been discharged from the reactor may be lost to the atmosphere. The premature discharge of unreacted particles is referred to as "breakthrough" and, if the reaction kinetics are known, the extent to which breakthrough occurs can be estimated through calculations about an ideal CFSTR.
Practical evidence as well as theoretical calculations of CFSTR kinetics shows that this breakthrough can be significant. While the actual amount of breakthrough is affected by the particle size of the carbide feed, the hydrodynamic behavior of a carbide particle reacting to form acetylene, the internal configuration of the reactor and the inherent reactivity of the calcium carbide, the amount of breakthrough increases as the space velocity of the reactor increases. Space velocity is defined conventionally as the number of reactor volumes displaced in one hour.
FIG. 1 is a graph showing the mathematical relationship between the space velocity and breakthrough, assuming ideal behavior and relying on published data for CFSTR kinetics. For reactors of space velocities equaling four or greater (4000 liter/hr. in FIG. 1), the ideal conversion is about 96% or less, which means that the breakthrough losses are greater than 4% of the acetylene that is produced from a given carbide feed. Most commercial reactors in operation today operate at a space velocity greater than four, which means that the breakthrough losses are even greater.
Dissolution Losses
Another problem with traditional technologies is the losses of acetylene to dissolution in water. FIG. 2 shows the solubility of acetylene in water as a function of temperature for three commonly operated pressures. It may reasonably be assumed that the water of the hydrated lime discharge slurry is saturated in acetylene, and unless this water is recycled to the system, all of the acetylene dissolved in the water will eventually be lost to the atmosphere. The amount of acetylene contained in the discharge hydrated lime slurry can be calculated by knowing the amount of water exiting the reactor and its outlet temperature and pressure. Even if some of the water is recycled to the reactor the open vessels which serve as settling tanks to thicken the hydrated lime are exposed to the atmosphere and a large portion of the acetylene so dissolved is lost.
To illustrate, if pure calcium carbide (MW=64) is reacted to form acetylene (MW=26) with enough water to result in a 5% hydrated lime (MW=74) slurry, the amount of water flowing from the outlet per Kg of acetylene produced will be: ##EQU1## Assuming that the outlet temperature is 50.degree. C. and the reactor pressure is 0.3 atm-gauge (a typical set of conditions), the amount of acetylene contained in 54 Kg of water is 0.09% or 0.05 Kg. Thus about 5% of the acetylene generated is lost to the atmosphere through the dissolution in water. This loss is in addition to the acetylene lost as a result of breakthrough of unreacted particles.
Operational Difficulties
Another problem with conventional technologies pertains to the operational difficulties created when non-reactive materials are introduced along with the calcium carbide. These unreactive materials, which are present in all commercially available calcium carbide materials, usually comprise inert coke, solid ferrosilicate and other metallic or mineral particles. These are materials introduced with the limestone or coke fed to the furnaces that manufacture calcium carbide and are carried through to the final product. These inert materials accumulate in the reactor and if not removed will eventually interfere with the stirring mechanism and discharge pumps or valves, causing mechanical breakdown. Smaller particles that do not rapidly settle are carried through to the settling tanks where they may accumulate, causing difficulties with the discharge system.
Loss of Process Efficiency
Another problem of traditional systems is that the concentration of hydrated lime in the reactor is kept low, usually below about 10 weight percent, to reduce premature settling of hydrated lime, i.e., to keep the slurry of the hydrated lime precipitate in a free-flowing state. If allowed to settle the hydrated lime would result in plugging of discharge lines or create unmanageable accumulations of hydrated lime in the reactor, or both. Low hydrated lime concentrations also result from the manner in which temperature is controlled. In a typical process, temperature is controlled at 50.degree. C. If the temperature begins to rise, the usual procedure is to increase the rate of water feed, which brings down the temperature, but also has the undesirable result of further diluting the hydrated lime output stream. In hot weather, where heat losses to the environment are reduced, and for very large systems, the hydrated lime concentration in the output stream can fall to as little as 3 weight percent.
The operational requirement for low concentrations of hydrated lime requires larger reactors for a given space rate or a higher space rate for a given reactor size. Larger reactors require greater capital costs. Higher space rates result in greater breakthrough. Low hydrated lime concentrations also result in a greater per unit water throughput increasing the losses due to solubility. The combined losses due to these effects are typically 8 to 12%, which means that only 88 to 92% of the acetylene produced by a given calcium carbide is recovered. Finally, the resulting hydrated lime discharge stream must be sent to larger holding vessels in order to provide adequate residence time for the hydrated lime to thicken for subsequent use or disposal. These vessels occupy more land and require the need for additional capital investment.
Loss of Calcium Hydroxide Product Value
Particulate impurities contained in the calcium carbide are carried into the hydrated lime product unless they are filtered out at some expense and operational effort. These small particle impurities may adversely affect the value of the hydrated lime for downstream utilization. In general, the value of hydrated lime improves if the granular impurities are reduced in both size and quantity. The value of hydrated lime also improves with concentration and with consistency in concentration. Most downstream uses require the transportation of the hydrated lime and the greater the concentration, the more Ca(OH).sub.2 is transported per ton of slurry. In commercial practice of the prior-art, little if anything is done to segregate small particle impurities. In addition, processes to increase the concentration of the hydrated lime are separate from the reactor and add additional capital and operating costs.
Objects of the Invention
An object of the present invention is to reduce the aforementioned problems of a) loss of acetylene due to breakthrough, b) loss of acetylene due to solubility, c) operational difficulties due to inert materials, d) loss of process efficiency due to low hydrated lime concentrations, and e) loss of hydrated lime value due to particulates and low, or variable, hydrated lime concentrations.
It is also an object of the present invention to provide an apparatus and a process for reacting calcium carbide with water that provides a high level of recovery of the acetylene produced.
It is also an object of the invention to improve the level of recovery of acetylene in the reaction of calcium carbide with water by reducing or eliminating breakthrough and minimizing the acetylene dissolved in the water discharge stream.
It is also an object of the invention to provide an apparatus and a process which allows for high completion of reaction and low breakthrough of calcium carbide in a given reactor space.
It is also an object of the invention to provide an apparatus and process which provides low losses due to dissolution of acetylene.
It is also an object of the invention to provide an apparatus and a process which segregates large particles from small particles thereby improving the operability of the process and the value of the co-produced hydrated carbide lime.
It is also an object of the invention to provide an apparatus and a process which improves the process efficiency by improved utilization of reactor space.
It is also an object of the invention to provide an apparatus and a process which provides for control functions which allow for flexible acetylene output while maintaining a steady-state hydrated lime concentration and constant temperature.
It is also an object of the invention to provide control of the pressure of the apparatus while allowing variable acetylene production and steady-state concentration of hydrated lime and constant temperature.
It is also an object of the invention to provide a process control strategy which provides for maintaining the preferred temperature and hydrated lime concentration conditions while varying acetylene output.
It is also an object of the invention to provide an acetylene/calcium hydroxide production process in which the calcium hydroxide product is uniform in quality (particle size, purity and concentration).
It is also an object of the invention to provide an apparatus and process in which the hydrated lime product can be recovered from the reactor at high Ca(OH).sub.2 concentrations.
It is also an object of the invention to provide an apparatus and process which minimizes the requirements for internal moving parts and as a consequence simplifies the construction and operation of the apparatus.
It is also an object of the invention to provide an apparatus and process which minimizes the number and size of ports, external fittings, and moving seals through the pressure vessel, and as a consequence simplifies the construction and improves the overall safety and operability of the apparatus and process.
Further objects of the invention will become evident in the description below.